Methods and systems for aligning master oscillator power amplifier systems

ABSTRACT

The present disclosure provides a method for aligning a master oscillator power amplifier (MOPA) system. The method includes ramping up a pumping power input into a laser amplifier chain of the MOPA system until the pumping power input reaches an operational pumping power input level; adjusting a seed laser power output of a seed laser of the MOPA system until the seed laser power output is at a first level below an operational seed laser power output level; and performing a first optical alignment process to the MOPA system while the pumping power input is at the operational pumping power input level, the seed laser power output is at the first level, and the MOPA system reaches a steady operational thermal state.

PRIORITY DATA

This application is a divisional application of U.S. patent applicationSer. No. 16/165,022, filed Oct. 19, 2018, which claims priority to U.S.Provisional Patent Application Ser. No. 62/589,198, filed Nov. 21, 2017,each of which is hereby incorporated herein by reference in itsentirety.

BACKGROUND

The electronics industry has experienced an ever increasing demand forsmaller and faster electronic devices which are simultaneously able tosupport a greater number of increasingly complex and sophisticatedfunctions. Accordingly, there is a continuing trend in the semiconductorindustry to manufacture low-cost, high-performance, and low-powerintegrated circuits (ICs). Thus far these goals have been achieved inlarge part by scaling down semiconductor IC dimensions (e.g., minimumfeature size) and thereby improving production efficiency and loweringassociated costs. However, such scaling has also introduced increasedcomplexity to the semiconductor manufacturing process. Thus, therealization of continued advances in semiconductor ICs and devices callsfor similar advances in semiconductor manufacturing processes andtechnology.

For example, semiconductor lithography processes may use lithographictemplates (e.g., photomasks or reticles) to optically transfer patternsonto a substrate. Such a process may be accomplished by projection of aradiation source, through an intervening photomask or reticle, onto thesubstrate having a photosensitive material (e.g., photoresist) coating.The minimum feature size that may be patterned by way of such alithography process is limited by the wavelength of the projectedradiation source. In view of this, extreme ultraviolet (EUV) lightsources and lithographic processes have been introduced.

One of the methods to generate EUV radiation involves use of ahigh-power laser source such as a master oscillator power amplifiersystem (MOPA). A MOPA system includes a seed laser as a masteroscillator and several power amplifiers including high gainpreamplifiers as pre-amplifier and high efficiency power amplifiers aspower amplifiers. In order to achieve the maximum output power, a MOPAfor EUV generation requires alignment to provide focused laser pulses.Due to the limited optical efficiency of the power amplifiers, a largeamount of energy input into the amplifiers become heat that can changethe refractive index of all gaseous and solid medium through which thelaser radiation passes, resulting in a phenomenon commonly referred toas “thermal lensing.” Thermal lensing causes the laser beam from theMOPA system to drift, giving rise to reduction of laser power output andsafety issues. Therefore, there is a great deal of interests in methodsand systems for efficiently and safely align MOPA systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagrammatic view of an EUV light (also referredto as EUV radiation) source system, in accordance with some embodimentsof the present disclosure.

FIG. 2 is an exemplary schematic diagrammatic view of an EUV lightsource system including a laser beam impacting a target droplet andgeneration of EUV light, in accordance with some embodiments of thepresent disclosure.

FIG. 3 is a schematic diagrammatic view of a lithography system, inaccordance with some embodiments of the present disclosure.

FIG. 4 is a flowchart illustrating a method for aligning a MOPA system,in accordance with some embodiments of the present disclosure.

FIG. 5 is a schematic diagram illustrating the changes of power inputinto the power amplifier, power output of the power amplifier, and thepower output of the seed laser over time in the method in FIG. 4 , inaccordance with some embodiments of the present disclosure.

FIGS. 6 a and 6 b are schematic diagrams illustrating alignments betweenan aperture and a laser beam of a MOPA system, in accordance withaspects of the present disclosure.

FIG. 7 is a flowchart illustrating another method for aligning a MOPAsystem, in accordance with some embodiments of the present disclosure.

FIG. 8 is a schematic diagram illustrating the changes of power inputinto the power amplifier, power output of the power amplifier, and thepower output of the seed laser over time in the method in FIG. 6 , inaccordance with some embodiments of the present disclosure.

FIG. 9 is a schematic diagram illustrating the changes of power inputinto the power amplifier, power output of the power amplifier, and thepower output of the seed laser over time in the method in FIG. 8 , inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Additionally, throughoutthe present disclosure, the terms “mask”, “photomask”, and “reticle” maybe used interchangeably to refer to a lithographic template, such as anEUV mask.

As the minimum feature size of semiconductor integrated circuits (ICs)has continued to shrink for better performance and cost-effectiveness,there has continued to be a great interest in photolithography systemsand processes using radiation sources with shorter wavelengths. In viewof this, extreme ultraviolet (EUV) light sources, processes, and systemshave been developed. Methods to produce EUV light include, but are notnecessarily limited to, converting a material into a plasma state thathas an element (e.g., xenon, lithium, or tin) with an emissionwavelength in the EUV spectrum. In one such method, often termed laserproduced plasma (LPP), the required EUV light can be produced byirradiating a target material, for example in the form of a droplet,with a laser beam emitted from a laser system. In accordance with itsvarious embodiments, the present disclosure is generally related tosystems and methods for preparing and aligning components in the lasersystem to reduce beam drifting due to thermal lensing effect.

Referring to FIG. 1 , illustrated therein is a schematic view of a EUVlight generation system 100. The EUV light generation system 100 isillustrative of an exemplary system that creates EUV wavelengthradiation, which can be delivered to a EUV lithography system 200, whichwill be further described later in FIG. 2 . In some embodiments, the EUVlight generation system 100 is embedded in and is integrated as a partof the EUV lithography system 200. In some embodiments, a EUV lightgeneration system 100 may include a laser produced plasma (LPP) EUVlight source. Thus, as shown and in some embodiments, the EUV lightgeneration system 100 may include a laser system 112 for generating anddelivering a laser beam 114 to a EUV vessel 116.

The laser beam 114 may be a series of pulses. In some embodiments, thelaser beam 114 includes one or more main pulses, and/or one or morepre-pulses. Suitable lasers generated by the laser system 112 mayinclude KrF, ArF, CO₂ lasers, and other appropriate lasers. As anexample, the laser system 112 may include a pulse laser device (e.g., apulsed gas-discharge CO₂ laser device) producing a laser radiation at9.2 um or 10.6 um, with direct-current (DC) or (radio frequency) RFexcitation, operating at a relatively high power (e.g., 20 KW or higher)and a high pulse repetition rate (e.g., 50 KHz or more) with a pulsewidth from about 30 ns to about 100 ns.

In the illustrated embodiment, the laser system 112 has a masteroscillator power amplifier (MOPA) configuration, which includes a masteroscillator (MO) 118 as a seed laser source and multiple stages ofpre-amplifier 119 and power amplifier (PA) 120. For that reason, thelaser system 112 can be referred to as a MOPA system 112. The MOPAconfiguration can be used for power and energy scaling in a lasersystem, as well as to control each stage separately, such that therequired energy, power and efficiency can be optimized. Using a masteroscillator 118, for example, the laser beam 114 with an extremely tightspectrum may be generated for high-numerical-aperture lenses at lowpulse energy. Using a power amplifier 120, for example, the laser beam114 can be further amplified, in order to deliver the EUV power levelsnecessary for the high wafer throughput. The master oscillator 118 isalso referred to as the seed laser 118 because the laser beam 114 sharesmany properties with the output of the seed laser 118. In a particularembodiment, the seed laser 118 is a Q-switched laser, which can emitenergetic pulsed beam at change of the quality factor of the opticalresonator. The preamplifier 119 and power amplifier 120 are alsoreferred to as the laser amplifiers 117 or laser amplifier chain 117 andcan be powered electromagnetically. In a particular embodiment, thelaser amplifier chain 117 is an RF pumped, fast axial flow, CO₂ laseramplifier, where fast axial flow refers to high gas mixture flow ratealong a longitudinal direction of a discharge tube.

The state-of-the-art laser amplifier chain 117 still suffers from lowoptical efficiency and as much as 95% or more of the power input intothe laser amplifiers 117 becomes heat. To dissipate this heat and tocool the laser amplifiers 117, the MOPA system 112, in some embodiments,includes a cooling vessel 121 that surrounds some surfaces of the laseramplifiers 117. In some embodiments, a coolant flows through the coolingvessel 121 to cool the laser amplifiers 117. In those embodiments, thecoolant enters the cooling vessel 121 via a coolant inlet 123 and exitsthe cooling vessel 121 via a coolant outlet 125. The coolant can bewater, an aqueous solution of organic compounds, an organic solvent, amixture of organic solvents, mineral oil, or synthetic oil. In someimplementations, the coolant exiting the cooling vessel 121 can flow toa heat exchanger (not shown) where the coolant gives up heat before itenters the cooling vessel 121 via the coolant inlet 123. A coolingsystem including the cooling vessel 121 and the heat exchanger can be aclosed system where the coolant gains heat from the laser amplifiers 117and loses heat to the heat exchanger. In some embodiments, when thecoolant flow rate exiting the cooling vessel 121 is fixed, coolanttemperature at the coolant outlet 125 can serve as an indication of themacro thermal state of the laser amplifiers 117 or the MOPA system 112.For example, a substantially steady coolant temperature at the coolantoutlet 125 over a period of time can indicate that the MOPA system 112has reached its steady thermal state. In some instances, after the MOPAsystem 112 reaches a steady thermal state, if the power input into thelaser amplifiers 117 increases, it can take 10 to 40 minutes before thecoolant temperature at the coolant outlet 125 reaches a new steadytemperature, indicating a new steady thermal state of the MOPA system112.

In some embodiments, the MOPA system 112 includes a safety interlocksystem 127. The safety interlock system 127 can include a plurality ofinterlock mechanisms that can be activated or triggered by an event thatis considered hazardous by a user or under government regulations. Insome implementations, the safety interlock system 127 can shut the MOPAsystem 112 down if any of its interlock mechanisms is triggered. In someembodiments, the safety interlock system 127 includes temperaturesensors that are arrangement in a shape of a ring. The ring oftemperature sensors, sometimes referred to as an aperture ring, can bepositioned along the path of the laser beam from MOPA system 112 to EUVvessel 116. In some implementations, if one or more of temperaturesensors in the ring of temperature sensors deviates from a rollingaverage temperature by a predetermined extent, the safety interlocksystem 127 can be triggered to shut down the MOPA system 112. After theMOPA system 112 is shut down for parts swap or upgrade or systemadjustment, it can take hours or even days for the MOPA system 112 to beoperational again. In the semiconductor manufacturing industry, any gapin manufacturing can be costly due to loss of production capacity,increase of opportunity cost, and scrapping of possible below-standardproducts. For that reasons, any unplanned activation of the safetyinterlock system 127 is highly undesirable.

In the EUV light generation system 100, the laser beam 114 may then bedirected, by a beam transport and/or focus system 122, to the EUV vessel116. The path along which the laser beam 114 travels through from theseed laser 118 into the EUV vessel 116 is defined as the laser beampath. The chamber represented by the box 122 of FIG. 1 may includevarious devices to perform various functions including beam transport,beam focusing, beam amplification, and/or other suitable functionality.

In various embodiments, the EUV vessel 116 also includes a dropletgenerator 124 and a droplet catcher 126. In some cases, the dropletgenerator 124 provides droplets 128 of a target material (such as tin ora tin compound, discussed further below) into the EUV vessel 116.

The EUV vessel 116 may include one or more optical elements such as acollector 130. In some embodiments, the collector 130 may include anormal incidence reflector, for example, implemented as a multilayermirror (MLM). For example, the collector 130 may include a siliconcarbide (SiC) substrate coated with a Mo/Si multilayer. In some cases,one or more barrier layers may be formed at each interface of the MLM,for example, to block thermally-induced interlayer diffusion. In someexamples, other substrate materials may be used for the collector 130such as Al, Si, or other type of substrate materials. The collector 130may be an ellipsoid-shape with an aperture (or opening) 132 at thecenter to allow the laser beam 114 to pass through and reach anirradiation region 134. Thus, in some embodiments, the laser beam 114passes through the aperture 132 of the collector 130 and irradiatesdroplets 128 generated by the droplet generator 124, thereby producingplasma at the irradiation region 134. In some embodiments, the collector130 may have a first focus at the irradiation region 134 and a secondfocus at an intermediate focus region 136. By way of example, the plasmagenerated at the irradiation region 134 produces EUV radiation 138collected by the collector 130 and output from the EUV vessel 116through the intermediate focus region 136. From there, the EUV radiation138 may be transmitted to an EUV lithography system 200 for processingof a semiconductor substrate. The generated EUV radiation 138 is anelectromagnetic radiation having wavelengths of around 50 nm or less(also sometimes referred to as soft x-rays). In an embodiment, the EUVradiation 138 includes a wavelength centered around about 13.5 nm.

The interaction between the laser beam 114 and the target droplet 128 isdescribed in greater detail below with reference to FIG. 2 . Referringto FIG. 2 , illustrated are diagrammatic view of portions of the EUVvessel 116, which provide further details that may be applied to the EUVlight generation system 100 of FIG. 1 . FIG. 2 shows a diagrammatic viewincluding the collector 130 and an entry of the laser beam 114 throughthe collector aperture 132 and incident upon a target droplet 128 at theirradiation region 134. The target droplet 128 may be spherical in shapeor ellipsoidal in shape. The material of the target droplet 128 mayinclude xenon, lithium, tin, indium, antimony, or tellurium, with anemission line in the EUV spectrum. In one embodiment, the target droplet128 may include tin or a tin compound. Example compositions include, butare not limited to, tin, SnBr4, SnBr2, SnH4, tin-gallium alloys,tin-indium alloys, tin-indium-gallium alloys or combinations thereof.The target droplet 128 may have a diameter of approximately 10 um to 500um. Through the amplification from multiple laser amplifiers 117, thelaser beam 114 may have a power level ranging from about 10 kW to 40 kW,such as 26 kW in an example, before irradiating the target droplet 128.Upon the irradiation, the material in the target droplet 128 isconverted by the laser beam 114 into a plasma state and emits EUVradiation, illustrated as the EUV radiation 138. It is noted that in anexemplary embodiment, the collector 130 may be approximately 24 inchesin diameter with a 4 inch diameter aperture 132 in the center. The EUVradiation 138 may be angularly distributed such that it is incident uponthe mirror surface of the collector 130. The EUV radiation 138 isfurther collected and focused by the collector 130 to a focal point,such as the focal region 136 in FIG. 1 .

It is noted that in FIG. 2 , the target droplet 128 is elliptical (alsoreferred as a “pancake” shape) in the cross-sectional view. In otherembodiments, the target droplet 128 may be approximately spherical. Theelliptical shape may be provided by introducing a pre-pulse of the laser(e.g., a CO₂ laser) from the seed laser 118 prior to the introduction ofa main-pulse of the laser beam 114. The pre-pulse may be used to shapethe target droplet 128 increasing the available surface area for impactwith a subsequent main pulse of the laser beam 114.

As previously noted, the EUV vessel described above may be used toprovide an EUV radiation source for a EUV lithography system 200. By wayof illustration, and with reference to FIG. 3 , provided therein is aschematic view of an exemplary lithography system 200, in accordancewith some embodiments. The lithography system 200 may also begenerically referred to as a scanner that is operable to performlithographic processes including exposure with a respective radiationsource and in a particular exposure mode. In at least some of thepresent embodiments, the lithography system 200 includes an extremeultraviolet (EUV) lithography system designed to expose a resist layerby EUV light. In various embodiments, the resist layer includes amaterial sensitive to the EUV light (e.g., an EUV resist). Thelithography system 200 of FIG. 3 includes a plurality of subsystems suchas a radiation source 202, an illuminator 204, a mask stage 206configured to receive a mask 208, projection optics 210, and a substratestage 218 configured to receive a semiconductor substrate 216. A generaldescription of the operation of the lithography system 200 may be givenas follows: EUV radiation from the radiation source 202 is directedtoward the illuminator 204 (which includes a set of reflective mirrors)and projected onto the reflective mask 208. A reflected mask image isdirected toward the projection optics 210, which focuses the EUV lightand projects the EUV light onto the semiconductor substrate 216 toexpose an EUV resist layer deposited thereupon. Additionally, in variousexamples, each subsystem of the lithography system 200 may be housed in,and thus operate within, a high-vacuum environment, for example, toreduce atmospheric absorption of EUV light.

In the embodiments described herein, the radiation source 202 may be theEUV light generation system 100 in FIG. 1 . As discussed above, thesource may generate the EUV light using laser produced plasma (LPP). Insome examples, the EUV light may include light having a wavelengthranging from about 1 nm to about 100 nm. In one particular example, theradiation source 202 generates EUV light with a wavelength centered atabout 13.5 nm. In some embodiments, the radiation source 202 alsoincludes a collector, which may be used to collect EUV light generatedfrom the plasma source and to direct the EUV light toward imaging opticssuch as the illuminator 204.

Upon receipt, light from the radiation source 202 is directed toward theilluminator 204. In some embodiments, the illuminator 204 may includereflective optics (e.g., for the EUV lithography system 200), such as asingle mirror or a mirror system having multiple mirrors in order todirect light from the radiation source 202 onto the mask stage 206, andparticularly to the mask 208 secured on the mask stage 206. In someexamples, the illuminator 204 may include a zone plate, for example, toimprove focus of the EUV light. In some embodiments, the illuminator 204may be configured to shape the EUV light passing therethrough inaccordance with a particular pupil shape, and including for example, adipole shape, a quadrapole shape, an annular shape, a single beam shape,a multiple beam shape, and/or a combination thereof. In someembodiments, the illuminator 204 is operable to configure the mirrors(i.e., of the illuminator 204) to provide a desired illumination to themask 208. In one example, the mirrors of the illuminator 204 areconfigurable to reflect EUV light to different illumination positions.In some embodiments, a stage prior to the illuminator 204 mayadditionally include other configurable mirrors that may be used todirect the EUV light to different illumination positions within themirrors of the illuminator 204. In some embodiments, the illuminator 204is configured to provide an on-axis illumination (ONI) to the mask 208.In some embodiments, the illuminator 204 is configured to provide anoff-axis illumination (OAI) to the mask 208. It should be noted that theoptics employed in the EUV lithography system 200, and in particularoptics used for the illuminator 204 and the projection optics 210, mayinclude mirrors having multilayer thin-film coatings known as Braggreflectors. By way of example, such a multilayer thin-film coating mayinclude alternating layers of Mo and Si, which provides for highreflectivity at EUV wavelengths (e.g., about 13 nm).

As discussed above, the lithography system 200 also includes the maskstage 206 configured to secure the mask 208. Since the lithographysystem 200 may be housed in, and thus operate within, a high-vacuumenvironment, the mask stage 206 may include an electrostatic chuck(e-chuck) to secure the mask 208. As with the optics of the EUVlithography system 200, the mask 208 is also reflective. As illustratedin the example of FIG. 3 , light is reflected from the mask 208 anddirected towards the projection optics 210, which collects the EUV lightreflected from the mask 208. By way of example, the EUV light collectedby the projection optics 210 (reflected from the mask 208) carries animage of the pattern defined by the mask 208. In various embodiments,the projection optics 210 provides for imaging the pattern of the mask208 onto the semiconductor substrate 216 secured on the substrate stage218 of the lithography system 200. In particular, in variousembodiments, the projection optics 210 focuses the collected EUV lightand projects the EUV light onto the semiconductor substrate 216 toexpose an EUV resist layer deposited on the semiconductor substrate 216.As described above, the projection optics 210 may include reflectiveoptics, similar to those used in the illuminate 204. In someembodiments, the illuminator 204 and the projection optics 210 arecollectively referred to as an optical module of the lithography system200.

In some embodiments, the lithography system 200 also includes a pupilphase modulator 212 to modulate an optical phase of the EUV lightdirected from the mask 208, such that the light has a phase distributionalong a projection pupil plane 214. In some embodiments, the pupil phasemodulator 212 includes a mechanism to tune the reflective mirrors of theprojection optics 210 for phase modulation. For example, in someembodiments, the mirrors of the projection optics 210 are configurableto reflect the EUV light through the pupil phase modulator 212, therebymodulating the phase of the light through the projection optics 210. Insome embodiments, the pupil phase modulator 212 utilizes a pupil filterplaced on the projection pupil plane 214. By way of example, the pupilfilter may be employed to filter out specific spatial frequencycomponents of the EUV light reflected from the mask 208. In someembodiments, the pupil filter may serve as a phase pupil filter thatmodulates the phase distribution of the light directed through theprojection optics 210.

As discussed above, the lithography system 200 also includes thesubstrate stage 218 to secure the semiconductor substrate 216 to bepatterned. In various embodiments, the semiconductor substrate 216includes a semiconductor wafer, such as a silicon wafer, germaniumwafer, silicon-germanium wafer, III-V wafer, or other type of wafer asdescribed above or as known in the art. The semiconductor substrate 216may be coated with a resist layer (e.g., an EUV resist layer) sensitiveto EUV light. EUV resists may have stringent performance standards. Inthe embodiments described herein, the various subsystems of thelithography system 200, including those described above, are integratedand are operable to perform lithography exposing processes including EUVlithography processes. The lithography system 200 may further includeother modules or subsystems which may be integrated with (or be coupledto) one or more of the subsystems or components described herein.

FIG. 4 is a flowchart illustrating a method 300 for aligning the MOPAsystem 112 according to various aspects of the present disclosure. Themethod 300 is merely an example, and is not intended to limit thepresent disclosure beyond what is explicitly recited in the claims.Additional operations can be provided before, during, and after themethod 300, and some of the operations described can be replaced,relocated, or eliminated for other embodiments of the method 300. Themethod 300 is described below in conjunction with FIGS. 1 and 5 .

Reference is now made to FIG. 5 , shown therein is a schematic diagram400 illustrating the changes of pumping power input (represented by line420) into the laser amplifier chain 117, laser amplifier output(represented by line 410), the signal power output (represented by line430), amount of misalignment between an aperture of the MOPA system 112and the laser beam 114 (represented by line 440), and system temperature(e.g. coolant temperature at the coolant outlet 125 in FIG. 1 ,represented by line 450) of the seed laser 118, during the process ofaligning the MOPA system 112 according to method 300 of the presentdisclosure. Please note that FIG. 5 , FIG. 8 and FIG. 9 illustratequalitative variations of input/output levels, amount of misalignmentand system temperature over time and that the vertical axes of FIGS. 5,8 and 9 are not drawn to scale and do not represent any quantitativerelationship among illustrated parameters. The pumping power input intothe laser amplifier chain 117 can be in the form of radio frequency (RF)power input, optical power input, direct current (DC) power input, orother forms of electromagnetic power input. In a particular example, thepumping power input can be an RF power input. At operation 302, themethod 300 ramps up a pumping power input (represented by line 420) intothe laser amplifiers 117 of the MOPA system 112 until the pumping powerinput 420 is at an operational pumping power input level 425. As shownin FIG. 5 , at operation 302 of the method 300, at time to, the pumpingpower input 420 into the laser amplifiers 117 experience a step increaseto level 425, which is the operational pumping power input into thelaser amplifier chain 117. As used herein, the operational pumping powerinput refers to the level of pumping power input into the laseramplifier chain 117 when the MOPA system 112 is at regular operation.That is, when the EUV light generation system 100 generates EUVradiation under a regular manufacturing condition, the pumping powerinput into the laser amplifier chain 117 is at the operational pumpingpower input level—the level 425. In some embodiments, the operationalpumping power input level 425 is between 300 kW and 900 kW.

At operation 304 of the method 300, a seed laser power output (alsoreferred to as a signal power output, represented by line 430) of theseed laser 118 of the MOPA system 112 is adjusted until the seed laserpower output 430 reaches a first level 431 below an operational seedlaser power output level 435. Similar to the operational pumping powerinput 425 into the laser amplifiers 117, the operational seed laserpower output level 435 refers to the signal power output level of theseed laser 118 when the EUV light generation system 100 is fullyoperational for manufacturing purposes. In some embodiments, the firstlevel 431 is in the mW range. In some instances, the first level 431 isbelow 100 mW. Compared to the operational seed laser power output level435, which is between about 100 W and 300 W, the first level 431 is lessthan about 0.003% or about 0.01% of the operational seed laser poweroutput level 435.

At operation 306, the method 300 aligns the MOPA system 112 while thepumping power input is at the operational pumping power input level 425,the signal power output of the seed laser 118 is at the first level 431,and the MOPA system 112 reaches a steady thermal state. Because thepumping power input is the most substantial source of thermal energy forthe MOPA system 112, by turning the pumping power input to its fulloperational level, the MOPA system 112 can quickly heat up. The heatgenerated by the pumping power input can be removed by the coolantflowing through the cooling vessel 121. A signal power output from theseed laser 118 can also remove energy from the MOPA system 112 when theexcited gain medium coherently transfers energy to photons of the signalpower output from the seed laser 118. Line 450 represents the systemtemperature that, in some instances, can be assessed by measuring thecoolant temperature at the coolant outlet 125 of the cooling vessel 121.When the heat generated by the pumping power input is dynamicallybalanced by the heat removed by the coolant and the coherent energytransfer to the signal power output, the MOPA system 112 is said to havereached a steady thermal state. As shown in FIG. 5 , when the signalpower output is at the first level 431, the system temperature mayexperience a change represented by line segment 451. Line segment 451starts off at a higher level at time to and then gradually turns lowerdue to cooling and energy transfer until it plateaus at a steady statetoward the end of the line segment 451. In some embodiments, whether ornot the MOPA system 112 reaches a steady thermal state can be identifiedby measuring the temperature at the coolant outlet 125. For example,when the coolant temperature at the coolant outlet 125 fluctuates withina predetermined range for a predetermined period of time, a controller140 (FIG. 1 ) can determine that the MOPA system reaches a steadythermal state. The predetermined range and predetermined period of timecan be determined empirically. In an embodiment, the predetermined rangehas a span of 5 to 10° C. and the predetermined period of time isbetween 15 minute and 60 minutes. In some implementations represented byFIG. 5 , the pumping power input 420 is ramped up to the operationalpumping power input level 425. In those implementations, the steadythermal state can be referred to as the steady operational thermal stateas the pumping power input is turned to the operational level. Theprocess of aligning the MOPA system 112 according to method 300 can bereferred to an optical alignment process. In some embodiments, theoptical alignment process is performed by adjusting one or more mirrors,reflectors, or other optical components at both input and output ends ofthe laser amplifier chain 117.

The optical alignment process can be assessed or guided by the alignmentbetween an aperture 470 of the MOPA system 112 and the laser beam 114,which is illustrated in FIGS. 6 a and 6 b . As used herein and in someembodiments, the aperture 470 of the MOPA system 112 represents asummation of all optical paths and apertures of the MOPA system 112. Aperfect alignment between the laser beam 114 and the aperture 470 isillustrated in FIG. 6 a , where a center 475 of the laser beam 114coincides with a center 480 of the aperture 470. FIG. 6 b illustrates amisalignment between the laser beam 114 and the aperture 470, where thecenter 475 of the laser beam 114 does not coincide with the center 480of the aperture 470. The amount of misalignment between the laser beam114 and the aperture 470 can be expressed as a separation A between thecenter 475 of the laser beam 114 and the center 480 of the aperture 470.As illustrated in FIG. 6 b , the center 475 is shifted from the center475 by a distance H along the −X direction and by a distance V along theY direction. The separation A may then be mathematically represented asa square root of a sum of a square product of the distance H and thedistance V. The goal of the optical alignment process is to achieve theperfect alignment shown in FIG. 6 a or an alignment within an acceptablemargin of error of the perfect alignment.

The change of the amount of misalignment (i.e. separation A) during theprocess of aligning the MOPA system 112 according to method 300 isrepresented by line 440 in FIG. 5 . At time to, the MOPA system 112 maynot be aligned, as illustrated by line segment 441. In some instances,the MOPA system 112 may be fully aligned when cold (i.e. without anyinput of the pumping power) but such cold-state alignment cannot bemaintained due to thermal lensing as the pumping power input 420 isramped up to the operational level 425. Referring to line segment 441,the amount of misalignment may gradually drop over time due to coolingby the cooling system and the aforementioned coherent energy transfer.After the MOPA system 112 reaches a steady thermal state, as shown inthe right hand portion of line segment 451, the MOPA system 112 mayundergo alignment at operation 306 of the method and the amount ofmisalignment in line segment 441 further drops.

Performing an optical alignment process to the MOPA system 112 when theseed laser power output 430 is at the first level 431 and the pumpingpower input 420 is at the operational pumping power input level 425provides benefits. When the pumping power input 420 is at theoperational pumping power input level 425, the MOPA system 112experiences substantially the same level, if not higher, of the “thermallensing effect” as it is in regular operation. That is, at operation 306of the method 300, the alignment of the MOPA system 112 takes intoconsideration the impact of the “thermal lensing effect,” namely, thethermally-induced changes in refractive indices of all gases and opticalelements in the MOPA system 112. Aligning the MOPA system 112 using thefirst level 431 of seed laser power output ensures the laser output(also referred to as the laser amplifiers 117 output, represented byline 410) of the MOPA system 112 is at a low level. By ensuring thelaser amplifier output level 410 of the MOPA system 112 at a low level,alignment at the operation 306 is less likely to cause abrupttemperature gradients in the laser amplifier 117, reducing theprobability of triggering the safety interlock system 127. By reducingthe probability to trigger the safety interlock system 127, thealignment at operation 306 can save valuable on-time of the EUV lightgeneration system 100, reducing manufacturing cost and down-time.

At operation 308 of the method 300, the signal power output 430 of theseed laser 118 is ramped up through a number of ascending stages 432,433 and 434 to an operational seed laser power output level 435. Withthe pumping power input 420 being ramped up to the operational pumpingpower input level 425, abrupt increase of the output power 430 of theseed laser 118 to the operational seed laser power output level 435 canresult in undesirable level of self-focusing due to transient thermallensing effect. For that reason, operation 308 of the method 300advantageously ramps up the signal output power 430 of the seed laser118 through a number of ascending stages. As shown in FIG. 5 , after theMOPA system 112 is allowed to reach the steady operational thermal stateand aligned at operation 306, the signal output 430 of the seed laser118 is increased stage-by-stage at times t₁, t₂ and t₃ to ever higheroutput levels at stages 432, 433 and 434. In the exemplary embodimentshown in FIG. 5 , the signal power output 430 of the seed laser 118 isramped up through four stages—431, 432, 433 and 434, with the signaloutput power at stage 431 being at the first level and the that at stage434 and the operational seed laser power output level 435. In theembodiment represented by FIG. 5 , the four stages 431, 432, 433 and 434are even stages with equal increase of signal power output betweenstages. However, the present disclosure envisions fewer or moreascending stages. In theory, operation 308 can include any number ofascending stages. Due to practical considerations and to save time,operation 308 can include 3 to 15 ascending stages.

Because higher signal power output from the seed laser 118 extract moreenergy from the MOPA system 112, the step increases of signal poweroutput 430 over the four stages 431, 432, 433, and 434 may lower thesystem temperature 450 at the beginning of each corresponding systemtemperature stages 451, 452, 453, and 454. As shown by line segments451, 452, 453, and 454, the initial drop of system temperature may bedynamically balanced by the cooling system until the system temperatureplateaus toward the ends of each stages.

At operation 310, the method 300 aligns the MOPA system 112 at each ofthe number of ascending stages. In the exemplary embodiments shown inFIG. 5 , the signal power output 430 of the seed laser 118 is ramped upthrough three stages 432, 433 and 434 in addition to stage 431, andalignment is performed at each of the stages 432, 433 and 434. It isnoted that with the pumping power input being set at the operationalpumping power input level 425, the laser power output of the MOPA system112 increases stage-wise with each of the stages 432, 433 and 434. Asthe laser power output 410 reaches higher and near the operational laseroutput level 415, it can become more and more difficult to align theMOPA system 112 without triggering the safety interlock system 127. Insome embodiments, the alignment performed at operation 306 includes awider range and can be referred to as a coarse alignment and thealignments performed at operation 310 include narrower ranges and can bereferred to as fine alignments or fine tuning. This coarse-to-finealignment during the progression of method 300 is also illustrated inthe changes of misalignment amount in stages 441, 442, 443, and 444 inline 440. As shown in FIG. 5 , the amount of misalignment at time t₁ issmaller than the amount of misalignment at time t₀; the amount ofmisalignment at time t₂ is smaller than the amount of misalignment attime t₁; and the amount of misalignment at time t₃ is smaller than theamount of misalignment at time t₂. With the decrease of initialmisalignment amount over the stages, the alignment needed to bring theMOPA system 112 to full alignment at each stage also decreases. In someembodiments, little or no alignment may be needed when the signal poweroutput 430 is increased to level 434.

FIG. 7 is a flowchart illustrating a method 500 for aligning the MOPAsystem 112 according to various aspects of the present disclosure. Themethod 500 is merely an example, and is not intended to limit thepresent disclosure beyond what is explicitly recited in the claims.Additional operations can be provided before, during, and after themethod 500, and some of the operations described can be replaced,relocated, or eliminated for other embodiments of the method 500. Themethod 500 is described below in conjunction with FIGS. 1 and 8 .

Reference is made to FIG. 8 . At operation 502, the method 500 ramps up,through a first number of sequential stages 621, 622, 623, and 624, apumping power input at the level (represented by line 620) into thelaser amplifier chain 117 of the MOPA system 112 (FIG. 1 ) until thepumping power input represented by line 620 reaches an operationalpumping power input level 625. Each of the subsequent-in-time stagesincludes a higher pumping power input level. For example, the pumpingpower input level of the stage 622 is higher than that of the stage 621.Similarly, the pumping power input level of the stage 623 is higher thanthat the stage 622 and the pumping power input level of the stage 624 ishigher than that the stage 623. In the embodiment represented by FIG. 8, the four stages 621, 622, 623 and 624 are even stages with equalincrease of pumping power input between stages. While there are foursequential stages 621, 622, 623, and 624 in the exemplary embodimentshown in FIG. 8 , the present disclosure envisions ramping up thepumping power input 620 through any number of sequential stages. Due topracticality and to save time, in some embodiments, operation 502 canhave 3 to 15 stages. Similar to method 300, method 500 seeks to alignthe MOPA system 112 while the MOPA system 112 experiences the “thermallensing” effect due to the pumping power input and while the laseroutput of the MOPA system 112 is low. That is to say, for each of thefirst number of sequential stages 621, 622, 623, and 624, the pumpingpower input level 620 should be as high as possible, provided thatself-lasing can be kept at a reasonable low level. Self-lasing canhappen when residual gain in the laser amplifier chain 117 is too highto be extracted by the signal power output from the seed laser and isnot desirable for that reason. In instances where there are multiplelaser amplifiers 117 that receive pumping power input from differentpump power sources, the pumping power input 620 stands for the totalpumping power input to all of the multiple laser amplifiers 117. Inaddition, in those instances, the pumping power input into each of thelaser amplifiers 117 remains unchanged or increases from one of thefirst number of sequential stages to the next of the first number ofsequential stages with a higher total pumping power input 620.

At operation 504 of the method 500, a signal power output (representedby line 630) of the seed laser 118 of the MOPA system 112 is ramped up,through a second number of sequential stages 631, 632, 633, and 634,until the signal power output 630 reaches an operational seed laserpower output level 635. Each of the subsequent-in-time stage includes ahigher signal power output level. For example, the signal power outputlevel of the stage 632 is higher than that of the stage 631. Similarly,the signal power output level of the stage 633 is higher than that thestage 632 and the signal power output level of the stage 634 is higherthan that the stage 633. In the embodiment represented by FIG. 8 , thefour stages 631, 632, 633 and 634 are even stages with equal increase ofsignal power output between stages. As described above, method 500 seeksto align the MOPA system 112 while the MOPA system experiences the“thermal lensing” effect due to the pumping power input and while thelaser output of the MOPA system 112 is low. That is to say, for each ofthe sequential stages 631, 632, 633, and 634, the pumping power inputlevel should be as high as possible, provided that self-lasing can bekept at a reasonable low level. In some implementations, the initialsignal power of seed laser output at the first sequential stage 631 isthe mW range. In some instances, the initial signal power output at thefirst sequential stage 631 is below 100 mW. In some embodimentsrepresented by FIG. 8 , each of the second number of sequential stages631, 632, 633, and 634 coincides in time with one of the first number ofsequential stages 621, 622, 623, and 624. In those embodiments, each ofthe second number of sequential stages and its corresponding stage inthe first sequential stage start at the same point in time and last thesame duration of time. As shown in FIG. 8 , stages 621 and 631 start att₀ and end at t₁; stages 622 and 632 start at t₁ and end at t₂; stages623 and 633 start at t₂ and end at t₃; stages 624 and 634 start at t₃and end at the same point in time. While there are four sequentialstages 631, 632, 633, and 634 in the exemplary embodiment shown in FIG.8 , the present disclosure envisions ramping up the signal power output630 through any number of sequential stages. Due to practicality and tosave time, in some embodiments, operation 504 can have 3 to 15 stages.

Similar to line 450 in FIG. 5 , line 650 represents the systemtemperature that, in some instances, can be assessed by measuring thecoolant temperature at the coolant output 125 of the cooling vessel 121.As the pumping power input 620 and the signal power output 630 increasesthrough respective stages, the system temperature 650 may graduallydecrease and finally plateau at each stage of 651, 652, 653, and 654.For example, at time to, the system temperature starts off at a highlevel due to pumping power input and gradually drops toward the end ofline segment 651. Similarly, at t₁, the system temperature starts off ata locally high level due to the step increase of the pumping power inputfrom level 621 to level 622 and gradually drops toward the end of theline segment 652. Line segments 653 and 654 may also follow the sametrend.

At operation 506, the method 500 aligns the MOPA system 112 at each ofthe second number of sequential stages 631, 632, 633, and 634. As thelaser power output (represented by 610) of the MOPA system 112 reacheshigher with increase of power input and output signal power of the seedlaser 118, it can become more and more difficult to align the MOPAsystem 112 without triggering the safety interlock system 127. In someembodiments, the alignment performed at stage 631 includes a wider rangeand can be referred to as a coarse alignment. On the contrary, thealignments performed at stages 632, 633 and 634 include narrower rangesand can be referred to as fine alignments or fine tuning. Thiscoarse-to-fine alignment during the progression of method 500 is alsoillustrated in the changes of misalignment amount in stages 641, 642,643, and 644 in line 640. As shown in FIG. 8 , the amount ofmisalignment at time t₁ is smaller than the amount of misalignment attime t₀; the amount of misalignment at time t₂ is smaller than theamount of misalignment at time t₁; and the amount of misalignment attime t₃ is smaller than the amount of misalignment at time t₂. With thedecrease of initial misalignment amount over the stages, the alignmentneeded to bring the MOPA system 112 to full alignment at each stage alsodecreases.

As described above with respect to FIG. 8 , the four stages 621, 622,623 and 624 are even stages with equal increase of pumping power inputbetween stages and the four stages 631, 632, 633 and 634 are even stageswith equal increase of signal power output between stages. In someembodiments represented by FIG. 9 , method 500 can be modified such thatboth the pumping power input and the signal power output have largerincreases in earlier stages than in later stages to further set apartcoarse alignment and fine adjustments. As shown in FIG. 9 , the pumpingpower input starts off at level 721. From level 721, the pumping powerinput 720 increases by an amount P1 to level 722, increases by an amountP2 to level 723, and then increases by an amount P3 to level 724. P1 isgreater than P2 and P2 is greater than P3. In some implementations, P1,P2 and P3 are not uniform or are different from one another. In someembodiments, level 721 in FIG. 9 is greater than level 621 in FIG. 8 .The signal power output 730 starts off at level 731. From level 731, thesignal power output 730 increases by an amount S1 to level 732,increases by an amount S2 to level 733, and then increases by an amountS3 to level 734. S1 is greater than S2 and S2 is greater than S3. Insome implementations, S1, S2 and S3 are not uniform or are differentfrom one another. In some embodiments, level 731 in FIG. 9 is greaterthan level 631 in FIG. 8 . The modification to method 500 shown in FIG.9 may allow more coarse alignment to be done in early stages (e.g.721/731, 722/732) as the system temperature fluctuates more and takeslonger to stabilize in early stages and finer fine adjustment to be donein later stages (e.g. 723/733, 724/734) as the system can reach a steadythermal state faster in later stages. The modification illustrated inFIG. 9 may allow for a more efficient alignment of the MOPA system 112.It is noted that FIGS. 5, 8 and 9 are not drawn to scale and changes ofeach line therein are relative in nature and only meaningful withrespect to the parameter it represents. Lines in each of FIGS. 5, 8 and9 share the same time line.

In some embodiments, the EUV light generation system 100 can include acontroller 140. The controller 140 includes hardware and software toexecute methods 300 and 500. In some embodiments, the controller 140 isconnected to a pumping power source, such as a RF power source, thatsupplies power to the laser amplifiers 117, a seed laser controller thatcontrols and energizes the seed laser 118 and a temperature sensordisposed at the coolant outlet 125. The controller 140 can ramp uppumping power input into the laser amplifiers 117, ramp up signal poweroutput of the seed laser 118, and monitor the temperature reading at thecoolant outlet 125 to determine if the MOPA system 112 reaches a steadythermal state.

The various embodiments described herein offer several advantages overthe existing art. It will be understood that not all advantages havebeen necessarily discussed herein, no particular advantage is requiredfor all embodiments, and other embodiments may offer differentadvantages. For example, embodiments discussed herein provide systemsand methods for aligning a MOPA system in steady thermal states, whichsystems and methods improve the reliability, precision andsustainability in aligning the MOPA system, prevent beam drifting, andincrease availability of the MOPA system.

Thus, one of the embodiments of the present disclosure includes a methodfor aligning a master oscillator power amplifier (MOPA) system. Themethod includes ramping up a pumping power input into a laser amplifierchain of the MOPA system until the pumping power input reaches anoperational pumping power input level; adjusting a seed laser poweroutput of a seed laser of the MOPA system until the seed laser poweroutput is at a first level below an operational seed laser power outputlevel; and performing a first optical alignment process to the MOPAsystem while the pumping power input is at the operational pumping powerinput level, the seed laser power output is at the first level, and theMOPA system reaches a steady operational thermal state. In another ofthe embodiments, the method includes ramping up, through a first numberof sequential stages, a pumping power input into a laser amplifier chainof a master oscillator power amplifier (MOPA) system until the pumpingpower input reaches an operational pumping power input level; rampingup, through a second number of sequential stages, a signal power outputof a seed laser until the signal power output reaches an operationalseed laser power output level; and aligning the MOPA system at each ofthe second number of sequential stages.

In yet another of the embodiments, the method includes ramping up,through a first number of ascending stages, a pumping power input into alaser amplifier chain of a master oscillator power amplifier (MOPA)system until the pumping power input reaches an operational pumpingpower input level; ramping up, through a second number of ascendingstages, a signal power output of a seed laser from an initial level toan operational seed laser power output level, wherein the initial levelis less than 100 mW; and aligning the MOPA system at each of the firstnumber of ascending stages.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for aligning a master oscillator poweramplifier (MOPA) system, comprising: ramping up a pumping power inputinto a laser amplifier chain of the MOPA system until the pumping powerinput reaches an operational pumping power input level; adjusting a seedlaser power output of a seed laser of the MOPA system until the seedlaser power output is at a first level below an operational seed laserpower output level; and performing a first optical alignment process tothe MOPA system while the pumping power input is at the operationalpumping power input level, the seed laser power output is at the firstlevel, and the MOPA system reaches a steady operational thermal state.2. The method of claim 1, further comprising: ramping up, through anumber of ascending stages, the seed laser power output of the seedlaser from the first level to an operational seed laser power outputlevel; and aligning the MOPA system at each of the number of ascendingstages.
 3. The method of claim 2, further comprising: adjusting the seedlaser power output of the seed laser of the MOPA system until the seedlaser power output is at a second level greater than the first level andbelow the operational seed laser power output level; and performing asecond optical alignment process to the MOPA system while the pumpingpower input remains at the operational pumping power input level, andthe seed laser power output is at the second level.
 4. The method ofclaim 2, further comprising: prior to performing the first opticalalignment process, performing a second optical alignment process to theMOPA system while the pumping power input is at a pumping power inputlevel less than the operational pumping power input level, and the seedlaser power output is at a second level less than the first level. 5.The method of claim 1, wherein the first level is less than 100 mW. 6.The method of claim 1, wherein the operational seed laser power outputlevel is between 100 W and 300 W.
 7. The method of claim 1, wherein theoperational pumping power input level is between 300 kW and 900 kW. 8.The method of claim 1, wherein when the pumping power input into thelaser amplifier chain is at the operational pumping power input leveland the seed laser power output of the seed laser is at the operationalseed laser power output level, a laser output level of the MOPA systemis at an operational laser output level between 20 kW and 40 kW.
 9. Amethod, comprising: ramping up, through a first number of sequentialstages, a pumping power input into a laser amplifier chain of a masteroscillator power amplifier (MOPA) system until the pumping power inputreaches an operational pumping power input level; ramping up, through asecond number of sequential stages, a signal power output of a seedlaser until the signal power output reaches an operational seed laserpower output level; and aligning the MOPA system at each of the secondnumber of sequential stages, wherein an amount of increase of thepumping power input between a stage and a subsequent stage of the firstnumber of sequential stages gradually decreases throughout the rampingup, wherein an amount of increase of the signal power output between astage and a subsequent stage of the second number of sequential stagesgradually decreases throughout the ramping up.
 10. The method of claim9, wherein each of the first number of sequential stages coincides, intime, with one of the second number of sequential stages.
 11. The methodof claim 10, wherein a pumping power input level of each of the firstnumber of sequential stages and a signal power output level of each ofthe second number of sequential stages are selected such that thepumping power input level is maximized while self-lasing is minimized.12. The method of claim 9, wherein the MOPA system is allowed to reach asteady thermal state at each of the first number of sequential stages.13. The method of claim 9, further comprising: irradiating, using thealigned MOPA system, a laser beam toward a target to generate extremeultraviolet (EUV) radiation; collecting and focusing, using a collector,the EUV radiation to serve as a radiation source for an EUV lithographysystem; and patterning, using the EUV lithography system, a photoresistlayer on a substrate.
 14. The method of claim 9, wherein the firstnumber of sequential stages and the second number of sequential stagescomprise an identical number of sequential stages.
 15. The method ofclaim 14, wherein the first number of sequential stages comprise 3 to 15stages.
 16. The method of claim 9, wherein the MOPA system is surroundedby a cooling vessel, wherein a coolant enters the cooling vessel at acoolant inlet and exits the cooling vessel at a coolant outlet.
 17. Themethod of claim 16, wherein a temperature of the coolant at the coolantoutlet during the ramping up of the signal power output of the seedlaser.
 18. A method, comprising: ramping up a pumping power input into alaser amplifier chain of a master oscillator power amplifier (MOPA)system until the pumping power input reaches an operational pumpingpower input level; adjusting a seed laser power output of a seed laserof the MOPA system until the seed laser power output is at a first levelbelow an operational seed laser power output level; performing a firstoptical alignment process to the MOPA system while the pumping powerinput is at the operational pumping power input level, the seed laserpower output is at the first level, and the MOPA system reaches a steadyoperational thermal state; ramping up, through a number of ascendingstages, the seed laser power output of the seed laser from the firstlevel to an operational seed laser power output level; and aligning theMOPA system at each of the number of ascending stages.
 19. The method ofclaim 18, wherein the operational pumping power input level is between300 kW and 900 kW.
 20. The method of claim 18, wherein the first levelis less than 100 mW.